1. Field of the Invention
This invention relates to a molding method and apparatus of silicon nitride ceramic products.
2. Description of the Related Art
According to the prior art, a slurry consisting of a mixture of ceramic powders consisting mainly of Si.sub.3 N.sub.4 and a liquid is filled into a mold and pressurized, the density of the ceramic particles inside the mold is increased, and the liquid in the slurry is rapidly absorbed by a porous mold material and is discharged. Accordingly, the ceramic particles inside the mold come into pressure contact and a molded article is produced (e.g. Japanese Patent Publication No. 42321/1990 and Japanese Patent Laid-Open No. 70101/1985). A molding method of ceramics which fills regulated ceramic raw materials into a porous mold and molds them while pressuring the materials under vibration is known (e.g. Japanese Patent Laid-Open No. 217208/1986 and No. 8203/1989).
When molding is effected by the conventional pressure cast molding method described above, solidification of the slurry occurs from the portion of a moisture permeable material of the mold. Therefore, the difference of the solidification rates occurs between the inside and outside of the molded article and finally, variance of the density occurs inside the molded article. Particularly, a molded article having a large thickness undergoes deformation such as warp and cracks due to this variance of density. As to a molded article having a complicated shape, a mold design must be made in accordance with the flowing direction of the slurry for the shape of the molded article. As to the slurry, the liquid quantity in the slurry is preferably as small as possible to attain a high density of the molded article and to shorten a solvent removing time from the molded article, but when the liquid quantity in the slurry is reduced, the viscosity of the slurry increases and eventually, the slurry goes to the gel state. Particularly, when ceramic powder consisting mainly of silicon nitride and having a mean particle diameter of not greater than 1 .mu.m, which ceramic powder has been investigated in the present invention, is slurried using water as a solvent, the above-mentioned phenomenon occurs in a high powder content because of the poor wettability of the ceramic powder to water. To cast-mold the slurry under such a high viscosity state or a gel state, an extremely high pressure force is necessary. Accordingly, a mold made of a material capable of withstanding such a high pressure becomes necessary and a molding apparatus becomes great in size from the aspect of installation. The problem of mixture of impurities into the product due to the wear of peripheral components also occurs during pressing at a high pressure.